The present invention generally relates to the field of pyrotechnic initiators, and more particularly to a pyrotechnic initiator having a bridgewire configured in an enhanced heat-sinking relationship with the adjacent ignition charge and/or header glass surface.
Pyrotechnic initiators have many uses in industrial and consumer applications. One important use is in triggering the inflation of airbags in motor vehicles. Significant efforts have been made in the automotive industry to reduce the cost of manufacturing reliable airbag initiators. One advance has been the use of liquids and slurries in loading pyrotechnic charges into the initiators. As shown in U.S. Pat. No. 5,686,691 to Hamilton et al. (which is incorporated herein by reference for its disclosure of slurry-loading except to the extent that it contradicts anything explicitly set forth here), it is known to load a slurry charge into a can and place the can onto a header assembly under a high consolidation force (e.g., 1500 psi) so that the charge consolidates and forcefully presses against the header surface and bridgewire. It is not always desirable, however, to consolidate such charges. For one thing, consolidation tends to require a highly flush glass surface (because the extremely fine bridgewires typically used are easily damaged if pressed against irregularities or voids in the surface of the glass), which generally requires a machining or grinding step.
It is also known to apply a non-consolidated ignition charge to a bridgewire that is raised above a glass surface, with the non-consolidated charge being dried to form a monolithic solid that encapsulates the bridgewire. However, the thermal conductivity of such ignition charges is typically markedly lower than that of the glass in the header. Consequently, because the bridgewire is spaced apart from the glass, it has less of a heat sink available to it. The availability of a heat sink is in turn important because of its effect on the firing performance of the initiator. Specifically, a firing current having at least a predetermined xe2x80x9call-firexe2x80x9d level and duration (e.g., 800 mA for 2 milliseconds at xe2x88x9235xc2x0 C.) applied to the bridgewire must resistively generate heat that is reliably (e.g., 99.9999% of the time with at least 95% confidence) sufficient to ignite the charge. It is also generally required that the application of current up to a predetermined xe2x80x9cno firexe2x80x9d level and duration (e.g., 200 mA for 10 seconds at 85xc2x0 C.) will reliably not result in the bridgewire generating sufficient heat to ignite the charge. The all-fire and no-fire levels of an initiator are in significant part determined by the degree to which the bridgewire can release heat (and it is also determined in part by where the heat is released, i.e., fully into the charge, or also into part of the header assembly) resistively generated in it. At a certain level, the resistive heat generated by the current flowing through the bridgewire cannot be released quickly enough to prevent an increase in the temperature of the bridgewire, which may in turn cause increased heat generation in the bridgewire and then ignition. Thus, the provision of an enhanced heat sink for the bridgewire predictably increases observed all-fire and no-fire levels.
Finally, it is believed that initiators have been made with a plastic sealed feedthrough and a bridgewire lying flush against the plastic with an unconsolidated charge placed thereon. It is believed, however, that such a configuration would not likely result in enhanced heat sinking, and in any case a plastic seal is undesirable as it is generally not as hermetic and robust as a glass seal.
In these regards, it is believed that heretofore a pyrotechnic initiator has not included a bridgewire configured in an enhanced heat-sinking relationship with the adjacent ignition charge and/or header glass surface without the necessity for a consolidation force to ensure robust contact between the charge and bridgewire.
In accordance with the present invention, an initiator includes a bridgewire that is configured in an enhanced heat-sinking relationship without the necessity of a consolidation force. The bridgewire may be in an enhanced heat-sinking relationship with the adjacent ignition charge and/or the header glass surface.